and monitoring

The surveillance of marine mammals is part of the National Surveillance Program NOVANA. We have the responsibility of national surveillance for three Danish marine mammals: the harbour porpoise, the harbour seal and the grey seal. Our main focus is to monitor both population development and distribution within these particular species. Since 1991, a vast amount of data has been collected in Denmark, in collaboration with other institutions, by using satellite tags, passive acoustic monitoring, and scans performed by planes and boats.

Surveillance scans take place every year in smaller areas. Every 10th year, Aarhus University participates in a larger European scan of the North Sea, including all inner Danish waters.

It can be more challenging to count porpoises compared to seals, due to the fact that porpoises live their entire lives in the water. Read more about the tagging of porpoises and seals under Telemetry and tagging.


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Counts by plane

Pencil drawing of seal surveillance by plane
Drawing of seal counts from plane, artist is unknown

Planes are used to count species that have colonies on land, like Danish seals. When we take high quality photos from small planes flying over the resting areas, we can use the photos afterwards to analyse and estimate the species’ population size. These surveillance scans take place during their molting and reproduction periods; in these periods, the seals are most likely to spend most of their time on land. The population estimations from the reproduction periods can be used to understand the productivity of the young, which gives more information about the well-being of the species.

Line transect

This surveillance method is often used for monitoring whale populations, either by plane or by boat. Every whale that is seen when following a random, but still representative, pattern of lines through the specific area is registered. Meanwhile, another observation group counts all the whales in the area that are not counted by the transect. With this data, a correlation of how many animals are actually counted by the primary observer is made.


National Monitoring and Assessment Programme

NOVANA is a collaboration between the Danish Nature Agency and the Danish Centre for Environment and Energy at Aarhus University. Every year a science report is conducted for all the marine areas in Denmark, including surveillance of our marine mammals. 


Science report no. 657: High density areas for harbour porpoises in Danish waters. Teilmann, J., Sveegaard, S., Dietz, R., Petersen, I.K., Berggren, P. & Desportes, G. 2008: National Environmental Research Institute, University of Aarhus. 84 pp. – NERI Technical Report No. 657.

Summary | Sammenfatning | Full report i pdf-format (7,196 kB)  | Full appendix (18,949 kB) | Appendix 1 (6,881 kB) | Appendix 2-3 (2,122kB) | Appendix 4-5 (9,621 kB) | Appendix 6 (416 kB)

Abundance survey of harbour porpoises in Kattegat, Belt Seas and the Western Baltic, July 2012. Signe Sveegaard, Jonas Teilmann & Anders Galatius. Note from DCE – Danish Centre for Environment and Energy, Aarhus University. Pp. 12. Full report

High-density areas for harbour porpoises (Phocoena phocoena) identified by satellite tracking. 2011Sveegaard, S., Teilmann, J., Tougaard, J., Dietz, R., Mouritsen, K.N., Desportes, G., Siebert, U., Marine Mammal Science 27, 230–246.

Acoustic surveys confirm the high-density areas of harbour porpoises found by satellite tracking. 2011. Sveegaard, S., Teilmann, J., Berggren, P., Mouritsen, K.N., Gillespie, D., Tougaard, J., ICES Journal of Marine Science 68: 929–936.

Spatial interactions between marine predators and their prey : herring abundance as a driver for the distributions of mackerel and harbour porpoise. 2012. Sveegaard, S.; Nabe-Nielsen, J.; Stæhr, Karl-Johan; Jensen, T.F.; Mouritsen, K.N.; Teilmann, J., Marine Ecology - Progress Series 468: 245-253.

Defining management units for cetaceans by combining genetics, morphlogy, acoustics and satellite tracking. 2015. Sveegaard, Signe; Galatius, Anders; Dietz, Rune; Kyhn, Line Anker; Koblitz, Jens C.; Amundin, Mats; Nabe-Nielsen, Jacob; Sinding, Mikkel Holger Strander; Andersen, Liselotte Wesley; Teilmann, Jonas. Global Ecology and Conservation 3: 839-850. Full report

Abundance of harbour porpoises (Phocoena phocoena) in the western Baltic, Belt Seas and Kattegat. 2014. Viquerat, S., Herr, H., Gilles, A., Peschko, V., Siebert, U., Sveegaard, S., Teilmann, J. Marine Biology 161, 745–754.